The present invention concerns improvements in weft feeders for fluid jet looms, of the type capable of measuring the weft yarn lengths being inserted into the loom shed, known as "measuring weft feeders".
A known weft feeder of this type comprises an electric motor which causes the rotation of a yarn winding arm, and a drum held stationary around which said arm, winds up the yarn into even turns, forming a weft yarn reserve, the length of which is controlled by means of suitable sensors. The loom nozzle, provided to launch the weft yarn into the loom shed, draws from the weft feeder drum the weft yarn length required for each weft insertion. This length is measured and controlled by the weft feeder counting the number of unwound turns by suitable means. As soon as the yarn at the outlet of the weft feeder, to be launched into the loom shed, has reached the predetermined length, it is stopped on the drum by means of a device comprising a rod adapted to engage the yarn at the outlet of the feeder. The rod is movable between a withdrawn rest position and a projecting working position in which the end of the rod moves into a cavity formed on the drum periphery for this purpose close to its free end, so as to stop the weft yarn by engaging the same as it unwinds from the drum. Preferably, the weft yarn stopping device is an electromagnetic device, wherein said rod projects from a movable core, which comprises a head of ferromagnetic material subject to the contrasting actions of at least an electromagnetic coil and of a return spring, allowing the rod to move alternatively into the projecting yarn stopping position and into the withdrawn rest position.
The rod stopping device of measuring weft feeders is, in most cases, quite valid and reliable. However, it sometimes happens that the weft yarn may slip away under the rod of the stopping device and move beyond the same, by wedging into the drum cavity housing the end of said rod and passing between the rod and the bottom of said cavity without stopping, or stopping with some delay, which irreparably alters the exactness of the measurement and even causes serious inconveniences to the working of the weft feeder and of the loom.
This phenomenon mostly occurs with yarns having a strong twist or a high rigidity, as they are being cut by the loom shears after weft insertion. It is mainly determined by the sudden change in weft yarn tension when cutting takes place, which causes longitudinal and transversal oscillations between the yarn stopping point on the measuring weft feeder and its cutting point on the loom. These oscillations cause jumping of the yarn, which thus wedges in between the end part of the stop rod and the bottom of the drum cavity, without stopping.
For this purpose, a small space is usually provided between the end of the stop rod and the bottom of the drum cavity, by forming the rod limit stop means directly inside the structure of the electromagnetic stopping device. This solution is more apt to guarantee the working of said stopping device.
When the weft yarn slips away, passing under the stop rod, there is of course a measuring error which generally produces a weft insertion longer than required or, in the case of a loom working with several colors the insertion of a weft length in the successive beating up together with the correct insertion of a different color.
Attempts have been made in the past to overcome this drawback, by providing for the free end of the stop rod to come in contact with an element positioned at the bottom of the drum cavity. This element has to be sufficiently elastic, so as not to form a limit stop, and sufficiently resistant to rubbing wear, caused both by the yarn trying to slip away and by the rod hitting against the yarn.
Nevertheless, by adopting this system, it is the one hand difficult to positively prevent any rebounds of the stop rod; which are obviously harmful from many points of view, and, on the other hand, it has turned out impossible to realize an element against which the rod should stop, being so strong as not to require replacement after short periods of use of the weft feeder.